In baseball, on deck refers to being next in line to bat. In a professional game, the batter who is on deck traditionally waits in a location in the foul territory called the on deck circle.

Being on deck only guarantees the batter will get a chance bat in the inning provided if there are fewer than two outs, and the number of outs plus the number of baserunners adds up to fewer than three. Additionally, the manager reserves the right to pull the on-deck hitter for a substitute at his discretion.

[edit] Significance in save situations

A relief pitcher who comes in to pitch when his team is ahead has the ability to earn a save if the tying run is either on base, at bat, or on deck, and he then finishes the game without giving up the lead.

[edit] On-deck circles

There are two on-deck circles in the field, one for each team, positioned in foul ground between home plate and the respective teams' benches. They are technically known as next-batter's circles. The on-deck circle is where the next scheduled batter, or "on-deck" batter, warms up while waiting for the current batter to finish his turn. The on-deck circle is either an area composed of bare dirt; a plain circle painted onto artificial turf; or often, especially at the professional level, made from artificial material, with the team's logo painted onto it.